Where I come from, the alternative is "It’s not brain surgery!"… In fact, a TV program called Brainiac pitted a bunch of rocket scientists against some brain surgeons to do a notional "difficult task"… unfortunately I didn’t see who won!

My friend and I now say "it’s not quantum mechanics!" after I failed my Electronic Systems Engineering degree by not being able to understand that particular subject (and therefore unable to complete the Semiconductors and Optical Electronics courses).

I think of "rocket surgery" as repairing a rocket, such as astronauts do on the ISS (using rocket loosely). That’s a lot harder than rocket science. I’ve heard it used in practice as repairing a system while it is in use. Consider repairing the atmosphere system on a space craft while you still want to breathe.

"Brain science" is understanding how the brain works. IMHO, that’s an order of magnitude harder than brain surgery.